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Progress on this project to date has been very good.  We have achieved important goals in the calibration of the new coralline P/Ca proxy for seawater phosphate.  We have now begun to apply the proxy to down-core coral records, and have found very intriguing evidence of a fundamental regime change in nutrient concentrations of the central Equatorial Pacific in the 1980’s, consistent with a connection, though with a 6-year lag,  to the 1976-77 Pacific climate regime shift.  The project has supported portions of the research of three female graduate students to date.  The following is a summary of our findings to date.

Findings:

1.    Coralline P/Ca measured in 10-12 samples per year over a 4-year record in a Pavona Gigantea sample from the Gulf of Panama, yields a correlation with annual upwelling-driven phosphate increases in surface water.   This proves in a semi-quantitative manner that the P/Ca proxy works in concept.  While details required a more broad scale correlation and direct comparison to contemporaneous measurements of seawater phosphate, this first data set, now published, provided the proof of concept that justified further analyses (LaVigne et al., GRL, 2008).

2.    We found a strong positive correlation (r = 0.95) between mean coral P/Ca and climatological mean surface ocean phosphate (PO4 SW) for two genera of surface-dwelling corals, Porites lutea and Montastrea sp., growing at widely distributed sites, supporting the global applicability of this approach.  We show that skeletal P/Ca in colonies from distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO4 SW) concentration.  The regression slope and y-intercept appear to be genus-specific.  These results show that the P/Ca proxy works in a consistent manner for corals growing in widely different nutrient regimes and in locations spread around the globe.

3.    We further tested the validity of P/Ca in Pavona gigantea and Porites lobata corals by comparing skeletal P/Ca records from multiple colonies grown at the same upwelling location in the Gulf of Panamá to a contemporaneous time-series record of surface water PO4 SW at this site.  Strong linear correlations were found between skeletal P/Ca and PO4 SW for both coral genera (r = 0.84 - 0.95).  Statistically significant linear regressions revealed good intercolony agreement in P/Ca response (5-12% about mean slope), supporting the hypothesis that surface-dwelling corals reliably record ambient seawater phosphate.  This study allows for confident application of the following multi-colony calibration equations to down-core P. lobata and P. gigantea records from comparable upwelling sites.

P/CaPorites lobata = (21.1 ± 2.4) (PO4 SW) + (14.3 ± 3.8)

P/CaPavona gigantea = (29.2 ± 1.4) (PO4 SW) + (33.4 ± 2.7)

The efficacy of the P/Ca proxy is thus supported by both broad scale correlation to mean surface water phosphate and regional calibration against fully contemporaneous seawater phosphate variations.  This result, and that of (2) above, are now in press in Geochimica et Cosmochimica Acta.

4.    A Gulf of Eilat Porites core gave P/Ca that varied over a narrow range from 4-12 µmol/mol and showed little relationship to the nearby seawater phosphate record.  We conclude that when phosphate is as low as it is in these waters (0.01-0.1 µmol/kg), coral P/Ca does not respond faithfully or other dependencies (e.g. P in food particles) start to control skeletal P.  We do see some increases in P/Ca lasting 1-2 years, around 1991, 1996, and 2002.  Interpretation of these variations is still underway through communications with our Israeli colleague Dr. Boaz Lazar.  Overall, the Eilat record suggests that Porites may not provide a good record of seawater phosphate at extremely low phosphate concentrations, but might record other environmental variables if these can be understood with further research.

5.    A Porites coral core from Christmas Island (2°N, 157°W) was analyzed for P/Ca over a period covering 1978-1995, and reveals an apparent mode shift in central Equatorial surface nutrient concentrations, occurring during the 1980’s.   This is a large effect, with seawater phosphate (inverted from P/Ca using the global Porites calibration discussed above) decreasing from ~1.2 µmol/kg in the late 1970’s to ~0.2 µmol/kg in the early 1990’s.  This is an absolutely novel finding, since monitoring of nutrients in this region did not begin until the late 1990’s.  Earlier studies of Pacific climate had suggested that the equatorial Pacific had shifted into a long-term El Niño type phase following the recognized regime shift of 1976-77 recognizable in the PDO and other indexes of Pacific climate.  This would suggest decreased upwelling and reduced nutrient supply.  While there is evidence from the Line Islands corals of warming and freshening in this region, the effects were relatively subtle compared to the unpredictably large decrease in surface water phosphate as inferred from coralline P/Ca.  Overall, the 1980’s appear to be a decadal-scale mode shift in nutrient supply, interrupted by shorter interannual variations associated with the El Niños of 1982-83 and 1987.  We also produced Ba/Ca data, from which we infer surface water dissolved Ba.  In this region, dissolved Ba variations appear to be dominated by the drawdown effect associated with high export production; upwelling strength has little effect as the increase in dissolved Ba in the water column occurs deeper than the core of the upwelling source waters, and substantially deeper than the phosphocline.  Thus Ba/Ca shows maxima associated with the low-procuctivity El Niño years.  However, Ba/Ca temporal minima stayed near the same value throughout the record, suggesting that the decadal-scale decrease in phosphate was not accompanied by a substantial change in productivity in this region.  We speculate that is because the region is Fe-limited, so that the earlier, higher phosphate concentrations went unutilized by the regional primary producers, and later reduction in the phosphate supply thus has little effect on productivity near the equator, but may have a large effect in regions further north and west.  This first evidence of decadal-scale variability in Equatorial Pacific nutrients is currently being prepared for submission to Nature.